We propose to assay lean body mass (LBM) and fat in a group of 300 pregnant women by potassium 40 counting. LBM represents the active metabolic component of the body mass. The neonates will be weighed and measured, and the placenta studied. Factors other than maternal size which are known to influence birth weight (gestational age, use of drugs, diabetes, etc.) will be assessed. It is known that maternal size influences birth weight, but it is not known whether this effect is due to differences in lean weight or differences in fat content. We propose to find out.